Inner Classes - Java

This section illustrates how inner classes can be used to simplify the code when using event adapter classes.
To understand the benefit provided by inner classes, consider the applet shown in the following listing.

Inner Classes

In Chapter 7, the basics of inner classes were explained. Here, you
will see why they are important. Recall that an inner class is a class defined within another class, or even within
an expression. This section illustrates how inner classes can be used to
simplify the code when using event adapter classes.

To understand the benefit provided by inner classes, consider the
applet shown in the following listing. It does
not use an inner class. Its goal is to display the string "Mouse
Pressed" in the status bar of the applet viewer or browser when the mouse
is pressed. There are two top-level classes in this program. MousePressedDemo extends Applet, and MyMouseAdapter extends MouseAdapter.
The init( ) method of MousePressedDemo instantiates

MyMouseAdapter and provides this object as
an argument to the addMouseListener( ) method.

Notice that a reference to the applet is supplied as an argument to
the MyMouseAdapter constructor. This
reference is stored in an instance variable for later use by the mousePressed( ) method. When the mouse
is pressed, it invokes the showStatus( )
method of the applet through the stored applet reference. In other words, showStatus( ) is invoked relative to
the applet reference stored by MyMouseAdapter.

The following listing shows how the preceding program can be
improved by using an inner class. Here, InnerClassDemo
is a top-level class that extends Applet.
MyMouseAdapter is an inner class
that extends MouseAdapter. Because MyMouseAdapter is defined within the
scope of InnerClassDemo, it has access
to all of the variables and methods within the scope of that class. Therefore,
the mousePressed( ) method can call
the showStatus( ) method directly.
It no longer needs to do this via a stored reference to the applet. Thus, it is
no longer necessary to pass MyMouseAdapter(
) a reference to the invoking object.

An anonymous inner class
is one that is not assigned a name. This section illustrates how an anonymous
inner class can facilitate the writing of event handlers. Consider the applet
shown in the following listing. As before, its goal is to display the string
"Mouse Pressed" in the status bar of the applet viewer or browser
when the mouse is pressed.

There is one top-level class in this program: AnonymousInnerClassDemo. The init(
) method calls the addMouseListener(
) method. Its argument is an expression that defines and instantiates an
anonymous inner class. Let’s analyze this expression carefully.

The syntax new
MouseAdapter(){...} indicates to the compiler that the code between the
braces defines an anonymous inner class. Furthermore, that class extends MouseAdapter. This new class is not
named, but it is automatically instantiated when this expression is executed.

Because this anonymous inner class is defined within the scope of AnonymousInnerClassDemo, it has access
to all of the variables and methods withinthe scope of that class. Therefore, it can call the showStatus( ) method directly.

As just illustrated, both named and anonymous inner classes solve
some annoying problems in a simple yet effective way. They also allow you to
create more efficient code.